1. Field of the Invention
The invention relates to the field of infrared cameras and in particular to infrared cameras employing a warm shield.
2. Description of the Prior Art
Thermal imaging cameras are based on the detection and discrimination of infrared (IR) radiation. The thermal imaging process is based on IR radiation that is emitted from all non-reflective objects, where the amount of IR radiation emitted increases as the temperature of the object increases. Thermal imaging cameras use special IR optics to focus IR radiation onto special IR detectors or IR detector arrays. To optimize the performance of these detector arrays a baffle or shield is used to limit incoming IR radiation to the IR radiation that is collected and focused by the IR optics, see FIG. 1. As shown in FIG. 1, the cold shield blocks or vignettes the detector array""s view to optical paths that do not come from the collection optics. For this shield to be effective, the shield""s IR signature has to be smaller to the IR signature that is blocked by the cold shield. This means the IR signature of the shield or baffle needs to be either smaller or more stable as compared to the IR xe2x80x9cnoisexe2x80x9d signature from the non-optical paths.
Historically, IR detector arrays required cryogenic cooling and the detector was placed in a dewar, which is essentially a thermos bottle with an IR transparent window. Since cooling was readily available, the general approach was to use a shield or baffle with a high emissivity ( greater than 98%), place this shield in the dewar directly above the IR detector array usually at either an aperture or a field stop to optimize the efficiency of the baffling and keep this shield at the same cryogenic temperature as the IR detector array. The result was the shield had a much smaller IR radiation signature as compared to unwanted stray IR radiation.
There are two basic limitations associated with the historic approach of using a As cold shield to baffle a thermal detector. The first limitation is some newer IR detector materials (e.g., ferroelectric, micro-bolometer, pyroelectric) do not require cooling so a refrigeration source is not readily available as a cooling source to cool the IR baffle or shield. There is a strong desire to eliminate cooling in the IR camera to reduce complexity, power consumption and/or size of the camera, and adding cooling just for the baffle would negate the benefits of using an IR detector that can operate without cooling.
The other limitation is a traditional cold shield is usually placed within the dewar assembly. This requires that the shield or baffle be small (less than an inch) and mechanically rigid. This limits the optics to operating with a fixed f number or f-stop. Limiting operations to a fixed f-stop is sometimes detrimental. A variable f-stop is often desired for using aperture-optimized zoom or variable magnification optics. A variable f-stop is also sometimes desired to limit exposure when the signal is exceedingly bright or when there is a desire to have a deeper depth of focus.
The traditional design alternative to high emissivity cold shield is a high reflectivity warm shield or stop. Since a traditional warm shield has a high reflectivity (xcfx81), this implies that is has a low emissivity (xcex5) since xcex5=1xe2x88x92xcfx81. A traditional warm shield has little, if any, IR emission. Instead it reflects IR radiation from other sources. The problem with a traditional reflective IR warm stop is that stray IR radiation can still reach the detector. It just has to reflect off the warm stop instead of illuminating the detector directly.
A simple analogy would be trying to keep the one end of a hallway dark by linings the wall and covering the windows with mirrors instead of painting the walls and windows black. The problem is if there is a window around the corner, that window could still illuminate end of the hallway by bouncing light off of the mirror lining in the hallway.
What is needed is some kind of noncryogenic means which can be used to avoid stray IR impinging on the detector, but without the reflective problems of the traditional warm shield.
The invention is an improvement in a camera for imaging light from a source comprising a detector element or a detector array. An uncooled light shield is interposed between the detector element and the source to block all light from reaching the detector element other than light directly transmitted from the source into the detector element without reflection from any portion of the camera. The efficiency is optimized if the light shield is placed at either a field or an aperture stop and/or the surface between the light shield and detector array is a specular reflector. The light shield has an interior surface oriented toward the detector element and an opposing exterior surface oriented toward the source. An array of retroreflectors is disposed on the interior surface.
In the illustrated embodiment the array of retroreflectors comprises an array of corner cube reflectors. The interior surface is highly reflective to light or has a low emissivity at least within the range of interest detected by the detector array. The light shield has an aperture defined therethrough which allows light to pass through the aperture and to directly impinge on the detector element without reflection within the camera. The improvement further comprises optics which are arranged and configured to collect light from the source and to focus the light on the detector element.
The illustrated embodiment contemplates that the detector element, light shield, optics and array of retroreflectors are operable at infrared frequencies of light. However, the invention is not limited to infrared use, but my be employed at any optical frequency.
The invention includes a method of providing a camera with the above design and further a method of collecting light and delivering it to an imaging detector using the combination of elements described above.
While the invention may have in part been described for the sake of grammatical fluidity as elements for the performance of functional objects, it is to be expressly understood that the claims are not to be construed as necessarily limited in any way by the construction of xe2x80x9cmeansxe2x80x9d or xe2x80x9cstepsxe2x80x9d limitations under 35 USC 112, but to be accorded the full scope of the meaning and equivalents of the definition provided by the claims whether by the judicial doctrine of equivalents or by statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.